PWM inverters are employed to convert DC power to AC power. In many applications, it is important to minimize harmonic distortion in the AC output power of the inverter. Such harmonic distortion can be minimized by proper selection of the PWM pulse pattern used to control the inverter. A PWM pulse pattern comprises a set of pulses which are used to control the switches of the inverter so that the output of the inverter, when filtered, will approximate a sinusoidal AC waveform. Each pulse has a leading edge switching angle and a trailing edge switching angle. Thus, if seven pulses are used to create one-half cycle of the output AC waveform, for example, these seven pulses have 14 switching angles.
It is known that the harmonic content in the output AC waveform of an inverter can be reduced by appropriate adjustment of the switching angles of the PWM pulses supplied to the switches of the inverter. For example, in Ellert, U.S. Pat. No. 3,883,792, a performance index is determined based upon the magnitudes of selected harmonics in the output of a converter. The switching angles of the PWM pulses supplied to the converter switches are selected so as to minimize this performance index. However, this system is only responsive to the magnitudes, but not the phase displacements, of the harmonics. If the loads coupled to the inverter remain balanced, load changes result in changes in the magnitudes, but not the phase displacements, of the harmonics in the inverter output voltage. On the other hand, in the case of unbalanced loads, a change in the loads changes both the magnitudes and the phase displacements of these harmonics. Thus, a system like that disclosed in the '792 patent wherein the phase displacements of the harmonics are not taken into account when adjusting switching angles can only be adequate when balanced loads are to be driven.
A system for adjusting the switching angles of PWM pulses supplied to the switches of an inverter is also shown in Glennon, U.S. Pat. No. 4,527,226. In this system, the output of an inverter is measured as a function of power factor. For each power factor, a set of switching angles is analytically derived to reduce the harmonic content in the output of the inverter. These sets of switching angles are then stored in a memory. Thereafter, during operation of the system, one of the angle sets is selected as a function of the power factor at the output of the inverter, and this set of switching angles controls the inverter to reduce the harmonic content in the inverter output. If the power factor changes, a different set of switching angles is chosen from the memory.
Since the PWM switching patterns of the '226 patent are stored in memory, only a finite number of sets of switching angles are available to regulate the output of the inverter. Accordingly, if the sensed power factor is other than one having a set of switching angles stored in the memory, the system selects a stored set of switching angles corresponding to the power factor closest to the sensed power factor such that the harmonic content in the inverter output is reduced. However, the harmonic content may still be considered undesirably large in this case.
Rather than rely upon a limited number of stored sets of switching angles, Shekhawat, et al., U.S. Pat. No. 4,635,177 discloses a method of on-line, real time generation of PWM waveforms based upon a depth of modulation value. The depth of modulation value is obtained by full-wave rectifying the actual output voltage from the inverter and subtracting a reference voltage therefrom. The reference voltage represents the desired inverter output voltage.
All of these systems assume a stiff DC link over which the DC power is conducted, i.e. a DC link where the DC voltage on the link has no ripple or substantial AC component. However, as a practical matter, the DC link is usually not stiff and the ripple on the DC link modulates the PWM pulses at the inverter output to produce additional harmonic content in the output of the inverter. Kirchberg, U.S. Pat. No. 4,961,130 recognizes the problems caused by ripple and other AC components on the DC link and undertakes a Fourier analysis at the output of the inverter to determine the Fourier coefficients of selected harmonics to be controlled in the output of the inverter. A stored pattern of PWM pulses is selected in accordance with the teachings of the '226 patent identified above, and the Fourier coefficients are used to advance or retard the switching angles of the selected pattern in order to substantially eliminate the controlled harmonics in the inverter output.
All of these systems are arranged to reduce the harmonic content in the phase-to-neutral voltages at the output of the inverter. Furthermore, either these systems ignore ripple on the DC input to the inverter entirely or perform a Fourier analysis only of the output of the inverter without also analyzing the voltage on the DC link.